The vehicle brake system for use in a vehicle having a brake booster for enhancing the brake force when a brake pedal is depressed is widely known. For example, Patent Literature 1 discloses an electric brake actuator (electric booster device) using an electric motor as a boosting power source. In the electric brake actuator disclosed in Patent Literature 1, a shaft member moving back and forth in response to manipulation of a brake pedal is used as a main piston, and a cylindrical member sheathing the shaft member (the main piston) is used as a booster piston, and a hydraulic brake pressure is generated on the basis of a driving force inputted from the brake pedal to the shaft member (the main piston) and a driving force given by the electric motor to the cylindrical member (booster piston) for enhancing the tread force.